The present invention relates to a sample hold circuit for a LCD (Liquid Crystal Display) driver, and particularly to that to be applied in a driver circuit of a high precision LCD wherein a video picture displayed from an analog signal supplied thereto can be optionally inverted horizontally or vertically.
In a high precision LCD, high frequency input signals, that is, serial analog RGB signals for example, are generally converted into several parallel signals of low frequencies in order to be processed under frequency limit of the LCD panel.
FIG. 5 is a block diagram illustrating a basic configuration of the high precision LCD, of which an example is reported in "Full-color Liquid-Crystal Display Products" by Nakajima et al, pp. 12 to 16, NEC Technical Journal Vol. 46, No. 10/1993.
The LCD of FIG. 5 comprises;
an analog interface LSI (hereafter abbreviated as AIF) 22 supplied with an analog signal 23 for outputting four source driving signals 31-1 to 31-4, the analog signal 23 representing one of three (RGB) analog signals to be supplied to the LCD, and others omitted in FIG. 5 PA1 a first and a second source drivers 29 and 30 for activating source lines 37-1-i, 37-2-i, 37-3-i and 37-4-i of an LCD panel 36 with the source driving signals 31-1 to 31-4, i being an integer from 1 to j, PA1 a gate driver 35 for activating gate lines of the LCD panel 36, and PA1 a controller 28 for controlling the AIF 22, the first and the second source drivers 29 and 30 and the gate driver 35. PA1 a first emitter coupling logic circuit having a first and a second transistor, coupled emitters of said emitter coupling logic circuit grounded through biasing means and each base of said first and said second transistor supplied with each of complimentary scanning signals; PA1 a second emitter coupling logic circuit having n transistors, coupled emitters of said second emitter coupling logic circuit connected to a collector of said first transistor, n being a positive integer; PA1 a third emitter coupling logic circuit having n transistors, coupled emitters of said third emitter coupling logic circuit connected to a collector of said second transistor; PA1 a shift register 6 for generating n timing pulses by shifting a start pulse synchronized with a dot clock, each i-th of said n timing pulses delayed by i clock cycle(s) from said start pulse and delivered to bases of i-th transistors of said second and said third emitter coupling logic circuits, i being a positive integer until n; PA1 a sample hold section 5 for dividing an analog signal into parallel signals having n sample hold units, each of said n sample hold units outputting a parallel signal held therein by sampling said analog signal when a sampling signal is delivered thereto; and PA1 a current amplifying section 4 having n current amplifying means supplied with a power supply, each i-th of said n current amplifying means delivering said sampling signal to corresponding i-th of said n sample hold units when current flows through an input line thereof connected to a collector of corresponding i-th of said n transistors of said second emitter coupling logic circuit 2 together with a collector of corresponding (n-i+1)-th of said n transistors of said third emitter coupling logic circuit 3.
The serial to parallel conversion above mentioned is performed in the AIF 22 consisting of a preprocessor 24, a shift register 25, a sample hold circuit 26 and an output circuit 27.
The analog signal 23 is preprocessed in the preprocessor 24 with processes such as clamping or so called .gamma. correction. The analog signal 23 is shifted by the clamping so that its black level corresponds to black level of signals to be sampled, and difference of electro-optical characteristic between the LCD and the CRT is compensated by the .gamma. correction here.
The analog signal 23 after preprocessed in the preprocessor 24 is sampled in turn by the sample hold circuit 26 and divided into four parallel signals in order to reduce signal frequency to that able to be displayed on the LCD panel 36 controlled by the first and the second source drivers 29 and 30. In the example of FIG. 5, the analog signal 23 of high frequency is divided into four parallel signals as follows.
The controller 28 generates a dot clock 11 of a high frequency (107.5 MHz, for example) synchronized with a horizontal synchronous signal extracted from the analog signal 23 and a start pulse 12 of 1/4 frequency of the dot clock 11 for regulating start timing of the dot clock 11. The shift register 25 supplies four sampling signals to the sample hold circuit 26 by shifting the start pulse 12 synchronized with the dot clock 11. With the four sampling signals, each having frequency of 26.9 MHz=107.5/4 MHz in the case and shifted with each other by one cycle of the dot clock 11, the analog signal 23 is sampled in turn and held to be output through the output circuit 27 as each of the four source driving signals 31-1 to 31-4.
In the output circuit 27, each of the four outputs of the sample hold circuit 26 is transfered into an alternation signal, alternating its polarity by every horizontal sweep for prolonging life time of liquid crystal in the LCD panel 36, and buffered to sufficiently low impedance for driving the LCD panel 36.
On the LCD panel 36, plural (j=320, for example) sets of four source lines 37-1-i, 37-2-i, 37-3-i and 37-4-i are ranged horizontally corresponding to horizontal pixels (1280=320.times.4 in the case). The source lines 37-1-i to 37-4-i of each i-th of the j sets are activated in order with the four source driving signals 31-1 to 31-4 respectively, controlled by the first or the second source driver 29, 30 according to a source driver control signal 38 supplied from the controller 28 synchronized with the horizontal synchronous signal. (In FIG. 5, connections among the four source driving signals 31-1 to 31-4 and the source lines 37-1-i to 37-4-i are expressed simplified in the first and the second source driver 29 and 30.)
The gate driver 35 activates gate lines (not expressed in FIG. 5) in order according to a gate driver control signal 39 generated by the controller 28 synchronized with vertical synchronous signal of the analog signal 23.
Thus, a video picture is displayed on the LCD panel 36 according to the analog signal 23 by switching each set of source lines 37-1-i to 37-4-i with the first and the second source drivers 29 and 30 at 1/4 frequency of the dot clock 11.
Heretofore, an example is described, wherein the analog signal 23 is divided into four parallel signals, namely the four source driving signals 31-1 to 31-4. However, the number of the parallel signals is not limited to four, it may be eight or other number considering number of horizontal pixels and frequency limit of the LCD panel.
When the analog signal 23 is to be divided into n (n being an integer not less than 2) parallel signals, it is sampled according to n sampling signals delivered from the shift register 25, and sampled signals at every (nm+k)-th clock pulse (k being a positive integer until n and m being an increasing integer) of the dot clock 11 are held to be output as k-th source signal 31-k for activating k-th left source line 37-k-i of each i-th of j sets each having n source lines 37-1-i to 37-n-i.
In this way, a high precision LCD can be provided regardless of frequency limit of the LCD panel 36 in the example of FIG. 5.
Now, consider a case where a video picture is desired to be inverted horizontally, and also vertically in some case, for example, in order to present the video picture to a person facing to the operator by turnning the LCD panel around its horizontal axis, or for displaying a mirror picture of the operator taken with a CCD camera situated at the LCD panel.
For the purpose, the source driver(s) is sufficient to be controlled inversely, that is to activate source lines from right to left for obtaining a mirror picture, in an LCD wherein input analog signal is directly supplied without divided into parallel signals. However, in a high precision LCD as above described, in a set of source lines 37-1-i to 37-n-i, the analog signal 23 is displayed from left to right in the order, even when the first and the second source drivers 29 and 30 are controlled to activate the source lines inversely from right to left, because each k-th source line 37-k-i is still supplied with each (nm+k)-th sample of the analog signal 23.
Therefore, also the sampling signals from the shift register 25 should be controlled inversely for obtaining mirror picture, for example, so that every (nm-k+1)-th sample is held to be output for k-th source signal 31-k.
For the purpose, a bidirectional shift register is applied in a prior art.
FIG. 6 is a block diagram illustrating a bi-directional shift register composed of four D-type flip-flops 44a to 44d for controlling a sample hold circuit 26 having four sample hold elements 47a to 47d of the prior art.
In FIG. 6, all of five switching elements 45 and 45a to 45d are controlled either of A side or B side according to logic of scanning direction signal 46. When the five switching elements 45 and 45a to 45d are controlled to A side, the start pulse 12 is supplied to the most left D-type flip-flop 44a of the shift register 25 and shifted rightwards to the D-type flip-flops 44b to 44d in the order clocked by the dot clock 11. So, the analog signal 23 is sampled first by the most left sample hold element 47a, which is followed rightwards by the sample hold elements 47b to 47d in succession. When the five switching elements 45 and 45a to 45d are controlled to B side, the start pulse 12 is first delivered to the most right D-type flip-flop 44d to be shifted leftwards, and the analog signal 23 is sampled from right to left by the sample hold elements 47d to 47a in the order.
Thus, with the bi-directional shift register 25 of FIG. 6, the analog signal 23 can be divided into four parallel signals 31-1 to 31-4 in the order or 31-4 to 31-1 in the reverse order according to logic of the scanning direction signal 46, in the prior art.
If the switching elements can be formed on the LSI chip in a MOS process, each switching element does not cost but a transfer gate composed of two transistors. However, in LCD driver circuits, principal elements should be designed with bipolar transistors, because the LCD panel needs a high driving voltage, and a high-speed operation is demanded in the AIF 22.
FIG. 7 is a circuit diagram illustrating an example applied in the switching elements 45a to 45d of the prior art of FIG. 6.
In the example, there are provided NPN transistors Q49 and Q50 composing an emitter coupling logic circuit. When base of the NPN transistor Q49 becomes at logic HIGH by complementary scanning direction signals 48 and 49 supplied to bases of the emitter coupling logic circuit, the NPN transistor Q49 becomes ON and the NPN transistor Q50 becomes OFF, which activate a second emitter coupling logic circuit consisting of NPN transistors Q51 and Q53, and inactivate a third emitter coupling logic circuit of NPN transistors Q52 and Q54.
The second emitter coupling logic circuit activated supplied with a constant current J5, the NPN transistor Q51 becomes ON when A side input signal 51 is higher than a reference voltage 53, making emitter potential of the NPN transistor Q53 higher than the reference voltage 53. So, the NPN transistor Q53 becoming OFF, its collector potential is shifted to a power supply voltage 54, controlling emitter potential of an NPN transistor Q55 supplied with another constant current J6 to the power supply voltage minus its base-emitter voltage Vbe to be output as an output signal 55.
When the A side input signal 51 is at logic LOW being lower than the reference voltage 53, the NPN transistor Q53 becomes OFF and the collector potential of the NPN transistor Q53 is shifted to low by potential difference generated by the constant current J5 flowing through a resistor R1 from the power supply 54, which makes potential of the output signal 55 to its potential level minus base-emitter voltage Vbe of the NPN transistor Q55, that is, to logic LOW.
On the other hand, B side input signal 52 connected to the third emitter coupling logic circuit of the NPN transistors Q52 and Q54 gives no effect to the output signal 55.
On the contrary, when the third emitter coupling logic circuit of the NPN transistor Q52 and Q54 is activated and the first emitter coupling logic circuit is inactivated by inverting the complementary scanning signal 48 and 49, logic of the B side input terminal 52 is reflected to the output signal 55 in the same way.
Thus, a switching element is composed with bipolar transistors in the prior art.
However, as above described, the switching element of the bipolar transistor needs a lot of transistors. So, the bi-directional shift register manufactured in a bipolar process costs wide chip space and large current dissipation in proportion to number of D-type flip-flops therein.
This is a problem.